19C1 Spot field test for the presence of soil carbonates with dilute HCl

Solutions of hydrochloric acid (HCl), commonly 1.0 M or 3.0 M, offer a rapid, qualitative way of indicating the presence of soil carbonates in the field (e.g. Soil Survey Division Staff 1993). The chemical reaction described in Method 19B1 applies. The test does not substitute for the rapid ‘effervescence class assessment’ coded as Method 19D1. For precise studies, more quantitative tests are recommended.

Reagents

1 M Hydrochloric Acid

Dilute 100 mL of 10 M HCl, mix and make volume to 1.0 L with deionised water.

3.0 M Hydrochloric Acid

Dilute 300 mL of 10 M HCl, mix and make volume to 1.0 L with deionised water.

Procedure

Bedrock and Bedrock Fragment Identification

Carefully add one or two drops of (initially) 1.0 M HCl to a small sample of bedrock fragment on a spot-plate, if necessary after the ‘specimen’ is scratched, broken, or powdered by a rock hammer to expose fresh material or to increase the surface area. If CO₂ effervescence is quickly apparent, this likely indicates the presence of limestone. If the reaction is slow and/or insignificant after waiting 2–3 min, repeat the procedure but substitute with 3.0 M HCl. Dolomite, if present, reacts slowly and to a lesser degree than limestone. Record the field observation, usually as presence or absent with respect to carbonates.

Carbonate Concretions (nodules, threads, masses, etc.)

Take prior action to break-up hard concretions, weathering rinds, or coatings of non-reactive material prior to testing. For example, crush the specimen to increase the surface area and to expose fresh mineral surfaces, noting that dolomite rarely forms as a secondary or authigenic (formed in place) mineral in soil. Carefully add one or two drops of (initially) 1.0 M HCl and subsequently, on a fresh specimen, a few drops of 3.0 M HCl. Use a spot plate and allow around 1–2 min for the HCl to react. Interpret as described for bedrock and bedrock fragments above and record the field observation/s, usually as presence or absent with respect to carbonates.

Carbonates in the Soil Matrix

Select a representative (<2 mm particle size) soil specimen, noting that carbonates may be primary, secondary, or both, while carbonates may occur in discrete locations such as along the faces of peds. A positive reaction confirms the presence of carbonates. The specimen can be placed on a spot plate and given 1–2 min to react (see Note 1). Initially use 1.0 M HCl and, if observations are inconclusive, use 3.0 M HCl. Record the field observation, usually as presence or absent with respect to carbonates.

Note

1.  As dolomitic carbonates react slowly; the reaction can be easily overlooked. Also, carbonates may occur in specific locations, such as along the face of peds. To capture this potential diversity, soil specimens from several locations throughout the profile should be tested.

Table 19.4. Effervescence classes used to describe the entire soil matrix using 1.0 M HCl. Ratings and approximate CaCO₃ equivalents (%) are adapted from Kalra and Maynard (1991).